Oh! I know this one.
Okay so I think I'm qualified to aid, being that I've been doing MMD for about a year and a half now.
MMM has significantly better animation tools than MMD, but the layout takes a good while to figure out.
MMD is less advanced, but you can charge right into it and make a basic animation in minutes.
ANYWAYS, how to download???
Visit this website to get the MMD download.
Download MikuMikuDance, the latest version of MMD, MME, MMD Stages, Accessories and much, much more! LearnMMD is the hottest MMD site on the web!
learnmmd.com
However, on first open, you'll be greeted with an error screen.
Easy fix, download Microsoft Visual C++ 2008 Redist Package and DirectX 9.
HOWEVER, the DL for C++ was taken off of Microsoft, last I checked, so you'll need to use Wayback Machine(However the redirect in the link above seems to work fine)
DirectX 9 is hard to find too, but the link above has a DL mirror. Make sure those are installed, and if things are still acting up, I'm more than happy to help!
Happy animating!